1. Field of the Invention
This invention relates to a pilot fuel injection system for an internal combustion engine and more specifically to a metering valve for such a system.
2. Description of the Prior Art
The advantages of fuel injection are well known. The degree with which the advantages are obtained is governed greatly by the accuracy and timing flexibility of the metering valve or valves in an injection system and ultimately by the cost of the metering valves and system for controlling the valves. As injection system metering valve for a compression ignition engine should meter the quantity of fuel demanded for engine speed and load, should meter an equal quantity to each cylinder at the optimum time and rate, and should sharply control injection pressure rise and fall to the injection nozzle to avoid nozzle dribble and after injection.
Several different basic types of fuel injection systems have been devised. The most successful of the basic types have been the common rail system and the jerk pump system. Many variations and combinations of the basic systems have also been devised. The basic common rail system employs a single pump for maintaining injection pressure to a commom header and one or more metering valves. The rate of fuel metering in such systems is a function of time, since injection pressure is constant. The basic jerk pump system employs one or more jerk pumps which provide both the injection pressure and the metering. The rate of fuel injection in such systems is relatively constant with respect to degrees of engine crankshaft rotation; however, this rate varies greatly with respect to time; and therefore, the pressure varies greatly with respect to engine speed.
During the past several years common rail systems have had decreasing success with compression ignition engines operating over a wide speed and load range. Compression ignition engines require high injection pressures. The known types of metering valves capable of accurately metering the high pressure fuel have required actuating forces which are relatively high and synchronized. Engine driven actuating mechanisms provided both. However, they also operate the valving members in the metering valves at speeds proportional to engine speed, i.e., increasing engine speeds cause increasing valving member speeds with respect to time, thereby undesirably reducing the quantity of fuel metered, since metering rate is a function of time in such systems. Varying the size of the metering orifice in the valve as a function of engine speed and load was one method of maintaining and/or increasing the quantity of metered fuel. This method was costly and complex, as were the many other methods tried. Common rail systems have had a rather high degree of success with spark ignition engines, since such engines require a relatively low pressure for manifold injection, whereby the conventional metering valves may be actuated by a solenoid producing relatively low forces.
Injection systems employing jerk pumps, which combine pumping and metering into a single unit, have had a high degree of success with diesel engines. Such systems may have one combined unit supplying several engine cylinders via a distributor or one unit per engine cylinder. In either case the unit often includes a piston and a bore defining a chamber which is expanded and contracted in response to reciprocating movement of the piston. The piston is reciprocated by an engine driven cam at speeds proportional to engine speed. A variable volume of fuel is trapped in the expanded chamber and then impulsively pushed to an engine cylinder in response to the piston moving in a direction contracting the chamber. Such units have several disadvantages. High forces are required to raise the trapped fuel volume to the high injection pressure required for a diesel engine. The drive train between the piston and the engine must be designed to withstand high torques. If variable injection timing is required, the drive train must include a sturdy phase change mechanism capable of withstanding the high torques. The high driving forces causes side loading of the piston, thereby accelerating wear of the piston and the bore. Injection pressures are lower than ideal at low engine speeds and higher than ideal at high engine speeds, since the piston is proportional to engine speed. Leakage of fuel from the trapped volume increases with decreasing engine speed. Rise and fall of the injection pressure is rather slow due to the cyclic pumping of the fuel by the piston.
Most fuel injection systems for compression ignition engines inject a single fuel charge per compression stroke; some systems, known as pilot or two stage systems, inject a small pilot or precharge of fuel early in the compression stroke. The pilot charge may be injected 30.degree. to 60.degree. bTDC and is followed by a main charge close to TDC. Methods and advantages of pilot injection have been described by many: Dr. P. H. Schweitzer in "What Can be Gained by Pilot Injection" Automotive Industries, Vol. 79 (1938) pp 533-534; Monnot et al in U.S. Pat. No. 2,966,079; and P. Eyzat in U.S. Pat. No. 3,439,655. Dr. Schweitzer's article points out some of the advantages obtainable with pilot injection, e.g., elimination of the characteristic diesel knock by reducing the rate of cylinder pressure rise per degree of crankshaft rotation, lower peak cylinder pressure, increased power output, and a reduction in fuel consumption per horsepower hour. Dr. Schweitzer also identifies a problem encountered when attempting to reduce the pilot fuel injection concept to practice; specifically, providing a fuel metering valve which accurately meters a stable pilot charge over the full operating range of the engine.
Since Dr. Schweitzer's contributions, researchers have confirmed the stated advantages of the pilot fuel injection concept and in addition have discovered that the concept can be used to reduce pollutant exhaust emissions, such as oxides of nitrogen, while retaining the stated advantages. However, the task of economically producing a pilot fuel injection system which provides an accurate and stable pilot charge has proven to be even more difficult than the task of producing a single charge injection system.
U.S. patent application Ser. No. 403,308, filed Oct. 3, 1973 now abandoned and assigned to the assignee of this application, discloses a solenoid actuated spool type valve capable of metering very small and accurate pilot and main fuel charges to the cylinders of an engine. The valve of application Ser. No. 403,308 employs the concept of metering fuel only while momentarily defining a continuous passage through the valve by traversing a passage in the spool across an outlet passage in the housing. The spool velocity is independent of engine speed and the velocity is preferably the same for all engine speeds and loads. Further metering is started and stopped without reversing the spool velocity by completely traversing the passage. The traversing concept allows very small and accurate metering of the fuel charges. The features of the same spool velocities for all engine conditions and not reversing direction of the spool during metering (i.e., traversing) allows the use of simple and inexpensive means to control movement of the spool. Further, since direction of the spool is not reversed during metering, spool actuating forces are maintained relatively low. This improves the wear life of the valve, since high actuating forces adversely effect long wear life.
U.S. patent application Ser. No. 603,078, filed Aug. 8, 1975 and assigned to the assignee of this application, discloses an improved means for controlling the amount of fuel metered by the valve of application Ser. No. 403,308.
The valve of this application discloses improved means for actuating and controlling the amount of fuel metered by the valves of applications Ser. Nos. 403,308 and 603,078.